The use of catalytic converters to reduce the amount of atmospheric pollution resulting from combustion engines and industrial operations has increased rapidly in the last 25 years. Catalytic converters can he found in a multitude of varieties depending on the end use of the converter, the cost structure of the particular market, the availability of materials and fabrication techniques, etc.
Many converter designs employ catalyst members either as monoliths or as a plurality of separate members. These catalyst members generally contain one or more catalytically active species located on and/or in a porous catalyst support material. The catalyst support material itself usually is in the form of a coating which has been applied to either a metal or ceramic substrate. Substrates are generally shaped in a fashion to maximize the available surface area for catalytic reaction while maintaining adequate mechanical properties. Catalyst members (especially metal members) are often corrugated in order to increase the available surface area provided. In other instances, the members may he in the form of honeycombs or other reticulated shapes.
In recent years, there has been a strong interest in improved converters for mobile internal combustion engine applications (especially for automobiles) to address the ineffectiveness of present day converters when the converter is cold (e.g., during the minutes immediately following engine startup). Many of these designs use thin metal foil members as substrates for catalyst members. Thin foils are preferred, in part, because they allow more surface area for catalytic activity per unit volume. Some of these improved converter designs also involve the use of electrical heating elements in close proximity to catalyst support-coated metal foil catalyst members. In these electrically heated designs, the catalyst support material may also act as an insulator to protect against short circuiting between the various metal foil members in the converter. Effective thin metal foil catalyst members are essential to the fabrication and performance of many of these "next generation" designs.
As noted in U.S. Pat. No. 5,272,876, catalytic exhaust device for automotive applications must be able to survive both rigorous quality control test and severe operating conditions. These conditions often involve exposure to temperatures on the order of 800.degree.-950.degree. C. Effective catalyst supports must be able to retain their surface area/porosity characteristics even on exposure to these high temperature conditions.
In conventional industry practice, catalyst support materials are most often applied by a washcoat technique where the substrate is dipped into a slurry which contains particles of the support material. The substrate is then removed from the slurry and the slurry coating on substrate is dried and/or calcined. Usually, the dipping process must be repeated in order to build up a sufficient catalyst support layer. Once an adequate support layer has been formed on the substrate, the catalytically active species (typically precious metals) would then be placed (often by impregnation using a precious metal-containing slurry) on and/or in the support layer.
The catalyst support layer should have a high amount of available surface area while being coherent and adherent to the substrate. Porosity characteristics are of importance in terms of the overall efficiency of the converter employing the catalyst member. Cohesion of the catalyst support material is also important where the catalyst member is to be exposed to gas flows, thermal shock, etc. over the service life of the engine.
For thin metal foil substrates, adhesion is especially important since the metal foil substrate is generally much more flexible than the catalyst support coating. Poor adhesion characteristics can result in delamination of the catalyst support layer from the foil during handling, assembly, or use. For mass market applications such as the automotive industry, even infrequent delamination can become an intolerable problem in terms of quality control and performance reliability.
While dipping is a widely practiced coating technique, the results it provides are often less than ideal. Due (at least in part) to surface tension of the slurry, the washcoat slurry applied by dipping preferentially collects in negative radius of curvature areas of the substrate. This preferential collection results in a catalyst support layer of non-uniform thickness on the substrate. The problem of non-uniform support layer thickness is accentuated where the catalyst member has regions with widely differing radii of curvature (e.g. a corrugated member). In such instances, the slurry will collect in any areas having negative radius of curvature (valleys or crevices) of the substrate. Non-uniformity of thickness can result in non-optimal use of the available surface area in the catalyst member, waste of precious metals in areas of excessive coating thickness, restriction of gas flow past the catalyst member in the converter, etc. Non-uniform coatings are also prone to delamination from the metal substrate by cracking or chipping.
Various efforts have been made to improve the results obtained by dipping. Often, expensive auxiliary ingredients are added to the slurry to improve its ability to coat the substrate. In some instances, the substrates themselves have been pretreated to make them more amenable to coating. These efforts have not been successful in overcoming the problem of non-uniform support thickness.
In the search for alternatives to dipping, some attempts have been made to use electrophoretic deposition to deposit catalyst support materials on metal substrates. To date, however, it is not believed that electrophoretic deposition has been used to produce a commercially viable catalyst support having acceptable characteristics (e.g. porosity, cohesion, adhesion, etc.) on a flexible metal substrate suitable for use in automotive exhaust applications. Thus, there remains a need for catalyst members having more uniform thickness of support material on a flexible metal substrate wherein the catalyst support has acceptable physical characteristics. A need also remains for improved and alternative processes to make catalyst members using metal substrates. These needs are especially acute for automotive applications.